Zero Knowledge Message Board

A public message board, hosted on a solidity contract, that uses ZK-SNARKS to allow a user to register their identity as a member of a group and post messages on behalf of the group without revealing their identity. Currently, this implementation is only semi-decentralized as it uses a central server to send the transactions on behalf of the user, but there are ways to change this with a little more effort.

Setup

Use Node v14. If you don't have it, do:

nvm install 14.15.3
npm -v # 14.15.3

Install dependencis

yarn

Run

Compile circuits and contracts

Get and link ptau files

curl https://hermez.s3-eu-west-1.amazonaws.com/powersOfTau28_hez_final_15.ptau --output ./pot15_final.ptau
curl https://hermez.s3-eu-west-1.amazonaws.com/powersOfTau28_hez_final_20.ptau --output ./pot20_final.ptau
mkdir ./circuits/pots/
ln ./pot15_final.ptau ./circuits/pots/pot15_final.ptau
ln ./pot20_final.ptau ./circuits/pots/pot20_final.ptau

Compile the circuits to create the witness and keys

yarn compile:dev hash-check 15
yarn compile:dev hash-check-bits 20
yarn compile:dev sig-check 20

Create the Verifier library with the SNARK verification keys

yarn compile:verifier

Compile the solidity contracts

yarn compile:solidity

Deploy Contracts and run the client

Local

Deploy the contracts

yarn deploy:local

Run the client watcher and http server

yarn dev

Ropsten

Rebuild contracts with compile:circom instead of compile:dev

echo `beacon="<YOUR RANDOM STRING HERE>"` >> .env
yarn compile:circom hash-check 15
yarn compile:circom hash-check-bits 20
yarn compile:circom sig-check 20

Deploy the contracts

yarn deploy:ropsten

Run the client watcher and http server

yarn prod

View localhost:8080

Circuits

Circuit Name	Private Inputs	Public Inputs	Outputs	Description
hash-check	x	hash	out	Checks if MiMC(x) = hash; outputs MiMC(x)
hash-check-bits	x (256-bit list)	hash	out	Checks if MiMC(x) = hash; outputs MiMC(x)
sig-check	publicKey	hashes, sig, message	None	Checks eddsa_verify(publicKey, sig, message) == true; checks MiMIC(publicKey) is in list hashes

'hash-check'

Inputs	Private	Type	Description
'x'	Yes	256-bit integer	MiMC hash pre-image
'hash'	No	256-bit integer	MiMC hash

'hash-check-bits'

Inputs	Private	Type	Description
'x'	Yes	256-bit array	MiMC hash pre-image
'hash'	No	256-bit integer	MiMC hash

'sig-check'

Inputs	Private	Type	Description
'publicKey'	Yes	256-bit	EdDSA public key
'hashes'	No	[]	Registered public key hashes
'sig'	No	2-by-256-bit array	EdDSA signature
'message'	No	312-bit array	binary representation of the MiMC hash of the message

Example use

import { babyJub, eddsa } from 'circomlib';
import mimc from 'client/utils/mimc';

function buffer2bits(buff) {
    const res = [];
    for (let i=0; i<buff.length; i++) {
        for (let j=0; j<8; j++) {
            if ((buff[i]>>j)&1) {
                res.push('1');
            } else {
                res.push('0');
            }
        }
    }
    return res;
}

const message = mimc(1234).toString().padStart(78, '0');
const msg = Buffer.from(message, "hex");

const prvKey = Buffer.from("0001020304050607080900010203040506070809000102030405060708090001", "hex");

const pubKey = eddsa.prv2pub(prvKey);

const pPubKey = babyJub.packPoint(pubKey);

const signature = eddsa.sign(prvKey, msg);

const pSignature = eddsa.packSignature(signature);

const aBits = buffer2bits(pPubKey);
const rBits = buffer2bits(pSignature.slice(0, 32));
const sBits = buffer2bits(pSignature.slice(32, 64));
const msgBits = buffer2bits(msg);

const sig = [rBits, sBits];

const hash = mimc(...aBits).toString();

const inputs = { publicKey: aBits, hashes: [hash], signature: sig, message: msgBits };

Add a circuit

Make a new directory in /circuits/ with the name of the circuit.

Copy the pot15_final.ptau file from /circuits/hash into the new directory.

In the new directory, create circuit.circom and input.json with the test inputs.

Run npm run compile CIRCUIT_NAME, if that doesn't work npm run compile CIRCUIT_NAME 20. If it complains about an env file in development, use compile-dev instead of compile. If the circuit and input produce a valid proof you should see OK.

The compiled circuit.wasm file will be in /circuits/circuits-compiled/CIRCUIT_NAME. The proof key circuit_final.zkey and the verification key verification_key.json will be found in /circuits/keys/CIRCUIT_NAME.

An example of creating and verifying a new proof in Node can be found in /client/prover.js.

Run ./solbuilder.js to generate Solidity from the contracts.

How it works

1. User generates EdDSA key pair (pk, sk) and sends the MiMC hash to the server H(pk).
2. To vote, the user first proves they're registered to the poll by sending a snark proving that they have a public key pk to one of the recorded MiMC hashes.
3. Then, the user sends an EdDSA signature of the vote and a snark proving that the signature was produced by the private key associated with the public key they just verified.

Valid sig = EdDSA(sk, msg) where pk = private2public(sk) and H(pk) is a recorded hash